The first constellation of satellites transmitting positioning signals was put in place for military applications by the American State (Global Positioning System or GPS) at the start of the 1980s. Since then, GPS signals have been used by professional civil applications (management of fleets of lorries, aids to aerial navigation, geodesic surveys, etc.), and henceforth for general-public applications (automobile navigation with onboard terminals and pedestrian navigation with terminals of smartphone type). Other constellations were put in place by the Russian State (GLONASS) and the Chinese State (Baïdou). A constellation of European satellites (Galileo) is undergoing deployment. Generally, these navigation systems are designated by the acronym GNSS (Global Navigation Satellite Systems).
The basic principle of aiding satellite-based positioning and navigation is the calculation by a receiver furnished with electronic processing circuits of position, velocity and time (PVT) data on the basis of electromagnetic signals of centimetric wavelength transmitted by satellites in orbit. The calculation of the PVT data by a receiver on the basis of the signals of the satellites is affected by numerous errors of various types: impact of the crossing by the electromagnetic signals of the various layers of the atmosphere (troposphere, ionosphere), errors due to the reflections of the signals on objects in the neighbourhood of the receiver (multipaths), clock errors, errors of the electronic processing circuits, etc. For military applications, these errors are corrected notably through the use of the properties of the signals transmitted on reserved carriers (P(Y) code of GPS). Specific means of multi-sensor processing and fusion are furthermore generally envisaged in order to guarantee precision and integrity of measurements intended for critical uses. But these solutions are restricted and expensive.
To meet the increasing need for precision in civil applications, diverse means have been developed to correct the main errors: acquisition of signals originating from several constellations, improvement of antennas to increase the robustness of reception, correlation loops in the receivers, differential GPS which calls upon fixed base stations which broadcast a reference signal making it possible to correct the errors, terrestrial networks for broadcasting correction information, fusion of the satellite data with data of the motion sensors onboard the receiver, or giving receiver trajectory information (mapping, terrain models), etc.
In particular, the errors due to the crossing of the ionosphere by the signals transmitted by the radionavigation satellites weigh heavily in the global toll of the positioning errors (4 standard deviations, according to the publication by A Angrisano et al., “Ionospheric models comparison for single-frequency GNSS positioning”, ENC 2011, 12/2011, http://pang.uniparthenope.it/node/64). Several types of techniques for correcting these ionospheric errors may be employed in the state of the art.
A dual-frequency receiver can thus use a linear combination of the pseudo-distances calculated on the basis of the signals of each of the frequencies. The ionospheric error being strongly correlated with the frequencies, it will be able to be eliminated by said combination. However, dual-frequency receivers are not yet widespread among the general public. Furthermore, the convergence time to a stabilized measurement is relatively long (possibly reaching as much as half an hour).
It is also possible to use, notably with receivers using a single frequency, corrections calculated by error models based on an estimation of the total electron content of the corresponding atmospheric layers ionized by the ultraviolet part of the solar radiation. One of the state of the art models is that of Klobuchar (“Ionospheric time-delay algorithm for single-frequency GPS users”, IEEE Transactions on aerospace and electronic systems, Volume AES-23, N.3, 325-331). However, the seasonal, daily and spatial fluctuations of the models are such that the calculations are complex and that it is difficult to attain a precision which can be guaranteed for a time bounded to a few seconds for calculating the corrections and making them available.
A method making it possible to guarantee at one and the same time fast convergence, precision and integrity is the acquisition of specialized signals containing corrections calculated on the basis of the differences between the known positions of fixed stations belonging to a network and the positions calculated on the basis of the navigation signals of a GNSS constellation. These so-called “augmentation” systems (Satellite Based Augmentation Systems or SBAS) have a coverage which is regional for the calculation of the corrections and for their broadcasting or local for GBAS (Ground Based Augmentation Systems). The operational systems comprise notably EGNOS in Europe (European Geostationary Navigation Overlay Service), and WAAS in the United States (Wide Area Augmentation System). These various SBAS systems require an infrastructure which is unwieldy and expensive in terms of investment and utilization, notably reference ground stations operating under guaranteed conditions of dependability and precision, an intensive calculation centre, communication links between the ground segment and a geostationary satellite-based telecommunications network, and specific receivers. Today, these costs limit the use of these services, and therefore the positioning precision that they afford, to critical (aerial navigation) or professional services.
Less expensive approaches consist in simultaneously using the code of the GNSS positioning signal and the phase of its carrier (optionally on two frequencies), the receiver being positioned in relation to a fixed station of known position so as to remove the cycle ambiguity regarding the calculation of pseudo-distance on the basis of the carrier phase. These approaches, which can have several variants, are known by the name Real Time Kinematics (or RTK). An RTK system can only operate with at least one fixed station and the differential positioning with respect to this station will be precise only in a radius of the order of 10 to 20 km around this station. It is therefore used only for professional applications whose integrity constraints and radius of coverage are less than those addressed by SBAS systems, by coastal maritime navigation and geodesic surveys.
A limitation which is common to the differential approaches of SBAS and RTK type is that of requiring the use of reference stations whose position is known very precisely and of calculation algorithms which process the entirety or the major part of the errors with equivalent precision. This limitation restricts the access of users of general-public terminals furnished with standard GNSS signal acquisition capabilities to greater positioning precision.